Constitutively active mutants

Receptors that remain in the activated state even in the absence of ligand are often known as constitutively active mutants (CAMs). The resulting disruptions in rhodopsin signaling also often result in alterations in the phosphorylation of rhodopsin by rhodopsin kinase (GRKl), the specialized GRK enzyme expressed in the retina that is largely responsible for rapidly desensitizing the receptor when it is exposed to light. 

The gene encoding the pj-adrenergic receptor displays a fair degree of polymorphism in the human population. Like the dopamine receptors, the p -adrenergic receptor variants are often relevant to pharmacogenetics. Constitutively active mutant (CAM) and loss-of-function (LOF) variants are in evidence. 

Akt activity is induced in a PI-3K-dependent manner immediately suggesting that the phosphorylated lipid products of PI-3K mediate the activation. Incubation of purified Akt with purified 3-phosphorylated phospholipids results in various extents of activation (44,46). These lipids, such as PtdlnsJP, PtdIns(3,4)P2> and PtdIns(3,4,5)P3, specifically associate with PH domains in a number of proteins (47). Furthermore, co-transfection of a dominant-negative form of PI-3K (delta-p85) also inhibits Akt activation (43). It was later shown that introduction of constitutively active mutants of the catalytic subunit of PI-3K was sufficient to activate Akt in cells (46,48). These studies strongly implicate Akt as a downstream effector of growth-factor-stimulated PI-3K activation in a variety of cell types. 

Parnot, C., Miserey-Lenkei, S., Bardin, S., Corvol, P., and Clauser, E. (2002). Lessons from constitutively active mutants of G protein-coupled receptors. Trends Endocrinol. Metab. 13, 336-343. 

Scheer, A., Fanelli, F., Costa, T., De Benedetti, P. G., and Cotecchia, S. (1996). Constitutively active mutants of the alpha lb-adrenergic receptor Role of highly conserved polar amino acids in receptor activation. EMBO J. 15, 3566-3578. 

Purohit A, Herrick-Davis K, Teitler M. Creation, expression, and characterization of a constitutively active mutant of the human serotonin 5-HT6 receptor. Synapse 2003 47 218-224. 

In addition to cell migration, another characteristic function of the VSM synthetic phenotype is proliferation. CaMKII has been implicated in cell cycle control in a number of systems, but investigations in this area have yield mixed results. Most studies to date have relied on pharmacological inhibitors of CaMKII such as KN-62 or KN-93 (Tombes et al. 1995) and point to a positive role for CaMKII in mediating the cell cycle. Conversely, overexpression of a constitutively active mutant of CaMKIIa. suggested a negative role for kinase (Beauman et al. 2003). Interpretation of the latter studies is complicated by potential nonspecific effects... 

In HEK cells coexpressing the delta opioid receptor and a constitutively active mutant of Gas and adenylyl cyclase type II, delta agonists cause Gfky-mediated stimulation of adenylyl cyclase type II [55]. Agonist action at delta receptors expressed in COS-7 cells or HEK 293 cells has been shown to stimulate the MAP kinase pathway, possibly mediated by Gfky subunits [56,57],... 

Epo receptor (EpoR) is not an oncogene. The EpoR gene is not significantly amplified or over expressed in solid tumors and over expression of constitutively activated mutant forms of EpoR does not transform cells... 

GTPase-deficient, GAP-insensitive, constitutively activated mutants. Based on mutations detected in RAS alleles found in human cancers. 

The mutation of a number of residues can result in the agonist independent activation of the receptor. These constitutively active mutants, a number of which are found in the C-terminal portion of il2, led to the hypothesis that GPCRs exist in equilibrium between two interconvertible allosteric states , R and R. Fanelli and coworkers recognised two drawbacks to the previous strategies used to identify agonist/antagonist induced structural changes ". The first drawback is that the... 

Experimental data and consideration of the crystal structure of rhodopsin suggest that intramolecular interactions stabilize the inactive conformation of GPCRs. Removal or rearrangement of these constraining interactions results in receptor activation as a result of movements of the TM helices, which are then relayed to the G protein-interacting intracellular loops. In the activated conformation, receptors display structural instability and enhanced conformational flexibility, as evidenced by the thermolability of constitutively active mutants (75). 

Lattion AL, Abuin L, Nenniger-Tosato M, Cotecchia S. Constitutively active mutants of the pradrenergic receptor. FEBS Lett 1999 457 302-306. 

Tryoen-Toth P, Decaillot FM, Filliol D, Befort K, Lazarus LH, Schiller PW, Schmidhammer H, Kieffer BL (2005) Inverse agonism and neutral antagonism at wild type and constitutively active mutant 5 opioid receptors. J Pharmacol Exp Ther 313 410-421... 

Rhodopsin(RHO) G90D, A292E, T4K, Retinitis pigmentosa, Constitutively active mutant ... 

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